The present invention relates to torque sensors and, more particularly. To non-contacting magnetoelastic torque sensors for providing a measure of the torque applied to a shaft.
In the control of systems having rotating drive shafts, torque and speed are the fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades.
Previously, torque measurement was accomplished using contact-type sensors directly attached to the shaft. One such sensor is a xe2x80x9cstrain gaugexe2x80x9d type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are very expensive and are thus commercially impractical for competitive use in many of the applications, such as automotive steering systems, for which torque sensors are now being sought.
Subsequently, non-contact torque sensors of the magnetostrictive type were developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis discloses a sensor comprising a torque carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically directed residual stress induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to equal, axial magnetizing forces. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used has created practical problems in applications where space is at a premium. Also, such sensors appear to be impractically expensive for use on highly cost-competitive devices, such as in automotive applications.
More recently, torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanent magnetizations have been developed which, preferably, utilize a thin wall ring (xe2x80x9ccollarxe2x80x9d) serving as the field generating element. See, for example, U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis. Tensile xe2x80x9choopxe2x80x9d stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed, uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a circumferential component and an axial component, the magnitude of the axial component depending entirely on the torsion. One or more magnetic field vector sensors sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space about the transducer and provides a signal output reflecting the magnitude of the torque. The stability of this transducer""s xe2x80x9ctorque-to-fieldxe2x80x9d transfer function under rigorous conditions of use reflects the efficacy of uniaxial anisotropy in stabilizing circular polarizations. This anisotropy, together with the spatially closed nature of the quiescent polarization, is also the basis of a striking immunity from polarization loss in relatively large fields. While the fields that arise from the ring itself have only hard axis components relative to the anisotropy, xe2x80x9cparasiticxe2x80x9d fields from permeable material that is close enough to become magnetized by the ring field have no such limitation. The addition of such parasitic fields to the torque dependent field from the ring can seriously degrade the near ideal features of the transfer function. As a result, in order to avoid a major source of such distortion, either the underlying shaft, or a sleeve that is placed between the shaft and the ring, is generally fabricated from a paramagnetic material. In addition, inasmuch as the peak allowable torque in a ring sensor is limited by slippage at the ring/shaft interface, concerns have been expressed regarding distortion arising from slippage at the ring/shaft interface under conditions of torque overload. This need for multiple parts of different materials, together with the requirement that the methods and details of their assembly establish both a rigid, slip-free mechanical unit and a desired magnetic anisotropy, have encouraged the investigation of alternative constructions.
Therefore, it is an object of the present invention to provide a magnetoelastic torque transducer in which the active, torque sensing region is formed directly on the shaft itself, rather than on a separate ferromagnetic element which then has to be affixed to the shaft.
It is another object of the invention to provide a magnetoelastic torque transducer in which the peak allowable torque is determined by the strength of the shaft material itself, rather than by slippage at an active element/shaft interface as in prior art sensors.
It is yet another object of the invention to provide a magnetoelastic torque transducer which depends upon the magnetocrystalline anisotropy of the shaft itself as the primary source of anisotropy for returning the magnetization to its previously established circumferential direction when the torque is reduced to zero, rather than upon techniques for instilling uniaxial magnetic anisotropy in the active, torque sensing region of a separate ferromagnetic element.
It is still another object of the invention to provide a collarless magnetoelastic torque transducer which depends for its operation on the sensing of a quantity that is inherently zero when the torque being measured is zero and which changes in both direction and magnitude in a correlative manner with the torque being measured.
It is yet another object of the invention to provide a magnetoelastic torque transducer which requires no external exciting field for its operation and which requires neither exciting currents nor coils.
A further object of the invention is to provide a magnetoelastic torque transducer including a unitary shaft of generally homogeneous chemical composition throughout comprising separate active and passive regions having magnetic properties appropriate for its respective function.
A still further object of the invention is to provide a non-contact method for measuring torque comprising the steps of providing a torqued member having a transducing region, magnetically polarizing the region in a single circumferential direction, the region possessing sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the member, to the single circumferential direction when the applied torque is reduced to zero, whereby a transducing region field arises which varies in response to torque on the member, and measuring a component of field output of the transducer as an indication of torque on the torqued member, the torqued member being formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90xc2x0 quadrant symmetrically disposed around the direction of circular remanence and having a coercivity sufficiently high that the transducing region field does not create parasitic magnetic fields in proximate regions of the member of sufficient strength to destroy the usefulness, for torque sensing purposes, of the net magnetic field seen by the field measuring apparatus.
Still another object of the invention is to provide a method for making a magnetoelastic torque transducer including a unitary shaft of generally homogeneous chemical composition throughout comprising separate active and passive regions having magnetic properties appropriate for its respective function by subjecting the appropriate regions of the shaft to thermal phase transformation processes, mechanical phase transformation processes, or combinations of thermal and mechanical phase transformation processes to obtain the respectively desired metallurgical phase in each such region in order to endow each such region with magnetic properties appropriate for its respective function.
These objects and others are achieved by providing a torque sensor comprising a magnetoelastically active region on a torqued shaft, and a magnetic field sensor, such as a Hall effect sensor, responsive to the transducing region field which arises as a result of the application of torque to the magnetoelastically active region. In the preferred embodiment, the magnetoelastically active region is polarized in a single circumferential direction and possesses sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the member, to the single circumferential direction when the applied torque is reduced to zero, the torqued shaft being formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90xc2x0 quadrant symmetrically disposed around the direction of magnetic polarization and having a coercivity sufficiently high that the transducing region field does not create parasitic magnetic fields in proximate regions of the shaft of sufficient strength to destroy the usefulness, for torque sensing purposes, of the net magnetic field seen by the magnetic field sensor. In particularly preferred embodiments the shaft is formed of a randomly oriented, polycrystalline material having cubic symmetry and the coercivity is greater than 15, desirably greater than 20 and, preferably, greater than 35.
In another embodiment of the invention these and other objects are achieved by providing a torque sensor including a unitary shaft of generally homogeneous chemical composition throughout comprising separate active and passive regions having magnetic properties appropriate for its respective function. Such a torque sensor is made by subjecting the appropriate regions of the shaft to thermal phase transformation processes, mechanical phase transformation processes, or combinations of thermal and mechanical phase transformation processes to obtain the respectively desired metallurgical phase in each such region in order to endow each such region with magnetic properties appropriate for its respective function.